The present invention relates to pressure sensitive adhesives comprising a synthetic latex and a tackifier resin.
Pressure sensitive adhesives are, among other applications, used in the tape and label field. A pressure sensitive adhesive may be used in a number of applications such as masking tape, electrical tape and medicinal tapes for holding dressings and the like in place. The term label covers an broad range of products and includes items such as price tags in the supermarket and labels on glass or plastic containers. In the case of "labels " two types of bonding are required namely permanent and removable. Permanent bonding is required for items such as labels on glass or plastic containers when the life of the label will be relatively long. Removable bonding is desirable for items such as price tags and peelable tape.
Pressure sensitive adhesives should have a high initial tack so that the adhesive surface merely needs to be contacted and pressed to a substrate to achieve bonding. There should be little or no requirement to hold the adhesive and substrate in position for any significant time while a bond develops. Depending upon the strength of adhesion required the bonding force of the adhesive may increase with time to provide a relatively permanent bond.
It is known to prepare at least three different types of adhesives using natural or synthetic tackifier, natural or synthetic resins, or blends thereof, namely, hot melt adhesives, solution adhesives and aqueous based adhesives. Hot melt adhesives are made from a blend of resins and are normally solid at room temperature. Hot melt adhesives require an elevated temperature for application to a substrate necessitating some type of hot melt gun.
Solution adhesives such as those described in U.S. Pat. Nos. 4,037,016, 4,048,124 and 4,248,748 do not suffer from the same drawback since the adhesives comprise a solution of a tackifier resin or a blend of tackifier resins in a low boiling hydrocarbon solvent. When the adhesive is applied to a substrate the solvent evaporates. Due to environmental concerns and the rising cost of hydrocarbon solvent there has been an increasing need to reduce or eliminate the hydrocarbon solvent in such adhesives. Accordingly, the adhesives industry is seeking improved aqueous based adhesive compositions. Existing aqueous based adhesive compositions usually comprise a natural or a synthetic latex in admixture with cellulose materials such as starch or natural or synthetic tackifier resins.
U.S. Pat. No. 4,189,419 to Avery International disclosed a pressure sensitive adhesive comprising from about 50 to 60% by weight on a dry basis of a tackifier and from about 50 to 40% by weight on a dry basis of a carboxylated styrene butadiene polymer (SBR).
A paper entitled "Styrene Butadiene Latexes for Adhesive Applications" by Robert G. Kahn of Dow Chemical, published in Adhesives Age, December of 1977 discloses a similar pressure sensitive adhesive composition which also contains butyl benzyl phthalate, as a "plasticizer". While the amount of plasticizer is relatively small the plasticizer comprises a hydrocarbon phase in the adhesive and the composition is not entirely aqueous based.
U.S. Pat. No. 4,179,415 issued Dec. 18, 1979, to Johnson and Johnson discloses a substantially aqueous based adhesive composition containing a latex having a high proportion of isoprene.
United Kingdom patent application 2097410A discloses novel pressure sensitive adhesives comprising a latex of a polymer and a tackifier resin the polymer being obtained from particular amounts of vinyl or vinylidene aromatic monomers and C.sub.4 -C.sub.8 conjugated diene monomers and an unsaturated carboxylic acid. Similarly European patent application publication number 0062343 A2 discloses a latex of a copolymer obtained from a hard monomer such as styrene, a soft monomer such as butadiene and an unsaturated acid which may be blended with a tackifying resin to produce a formulation useful as a pressure sensitive adhesive.
Various resinous materials have been suggested as tackifier for these latices. Examples of materials suggested include emulsified rosin, partially decarboxylated rosin, glyceryl esters of polymerised rosin, partially dimerised rosin, natural resins, hydrogenated wood rosin, plasticised hydrogenated rosin, aliphatic hydrocarbon resins from petroleum, aromatic petroleum resins, EVA, terpene/phenol resins, cumarone/indene resins, rosin esters, pentaerythritol esters and polydicylopentadiene resins.
In developing adhesive formulations one seeks the optimum combination of several adhesive properties and since it is not generally possible to achieve the optimum for every property it is necessary to obtain the best balance of properties for the particular use in mind. For pressure sensitive adhesives for use in the label industry one tries to optimise the loop tack and ball tack and at the same time provide an adhesive whose component will not migrate during storage leading to unsightly coloring and lowering of adhesive properties. Although the improved loop tack and ball tack can be provided by the tackifier resins of the type described above we have found that those tackifier which provide such tack for example hydrogenated rosins such as Staybelite ester 10 and modified rosins such as Snowtack 52CF tend to migrate.
It has been generally suggested in United Kingdom patent application 2097410A and European Patent application 0062343 that petroleum hydrocarbon resins such as those manufactured from a C.sub.9 cut of an aromatic hydrocarbon stream or a C.sub.5 cut of an aliphatic or cycloaliphatic hydrocarbon stream are useful as tackifiers. There is however no suggestion of the particular type of resin that should be used and indeed many of the resins falling within such broad descriptions are unsuitable for tackification of lattices.
Our European Patent Application 85302057.6 describes lattices of resins having a softening point from 10.degree. C. to 80.degree. C. which are copolymers of a feed which is predominantly C.sub.5 olefines and diolefines and one or more monovinyl aromatic compounds which contain from 10 to 30 wt. % of the monovinyl aromatic compound and their use as tackifiers for carboxylated styrene butadiene copolymer rubbers. We have now found that this type of copolymer is useful to tackify the polyacrylate emulsions frequently used in aqueous based adhesives.